Multicolor electrostatic printing



April 2, 1968 .1. ROTEMAN 3,376,133

MULTICOLOR ELECTROSTATIOPRINTING Filed Nov. 25, 1964 IMAGE TO BE PRINTED L* *1 YELLOW NEGATIVE COLOR H62 SEPERATION TRANSPARENCIES RED NEGATIVE COLOR SEPER- 9 FIG 3 T ATION TRANSPARENCIES NEGATIVE ELECTRO-STATIC .L K CHARGE Q CHARGED PLATE PLATE AFTER EXPOSURE To LIGHT THROUGH YELLOW F I65 NEGATIVE OF YEL LOW DEVELOPER PLATE AFTER EXPOSURE TO LIGHT THROUGH RED NEGATIVE RED - PLATE AFTER APPLICATION OF RED DEVELOPER INVENTOR. JEROME ROTE MAN BYE/060 M q- ATTORNEYS United States Patent al Ohio Filed Nov. 25, 1964, Ser. No. 413,832 6 Claims. (Cl. 96-1.2)

ABSTRACT OF THE DISCLOSURE A process of multicolor electrostatic printing which does not require repeated charging prior to application of each color. The process comprises (1) charging the photoconductive surface to a uniform potential, preferably negative, (2) exposing the charged surface to light through a negative color separation image transparency of a first color, (3) applying a developer corresponding to said first color and carrying a charge of the same polarity as that originally applied to the surface, and then, without recharging the surface, repeat steps (2) and (3) for each additional color that is to form a part of the multicolor print.

This invention relates to electrostatic printing and copying. More particularly, it relates to printing or copying of multicolored images.

One typical conventional electrostatic printing process includes coating a surface of a relatively conductive backing member with a photoconductive insulating material such as selenium, anthracene, or sulfur, and then providing an electrostatic charge of a given potential on the surface of the photoconductive coating. Then, a light image is focused on the charged surface discharging or lowering the potential of the irradiated areas, while leaving the remainder of the surface in a charged condition, thereby forming an electrostatic image. The electrostatic image may be rendered visible by the application of developer powder or liquid which is held electrostatically in the charged areas in the case of positive images, or, in the case of negative or reversal type images, the charge on the developer is such that the developer is repelled from the charged areas and deposits in discharged areas. The powder image is then transferred and fixed to another surface. A more detailed description of such a conventional process maybe found in US. Patent No. 2,297,691

to C. F. Carlson. A variation in the electrostatic printing process in which the intermediate step of transferring the powder image is eliminated and the developed or powder image is produced directly upon the desired surface is described in US. Patent 3,052,539 to H. G.

Greig.

In electrostatic printing of multicolor images, using the aforementioned printing techniques, it has been conventional to apply the first color as a colored developer and then to apply each subsequent color following the same procedure. For example, the surface of the photoconductive insulating material is charged with a potential of a selected polarity either negative or positive and exposed to light usually through a positive color separation image transparency which discharges or reduces the potential of the surface in the non-image areas of the particular color. (Color separation transparencies are conventional in both the color photography and color electrostatic printing arts. The multicolored material to be printed is conventionally reduced to a series of transparencies, one for each color. Such transparencies may either be positive, that is the image areas corresponding to a particular color are opaque to light while the non-image areas are transparent to light or they may be negative, that is, the image areas are transparent while the non-image areas are Patented Apr. 2, 1968 opaque.) Next, the charged developer having the desired color is applied to the exposed surface and optionally may be fixed to the surface. Then, for the next color to be applied, the entire procedure is repeated again. The sheet is again charged and exposed and optionally may be fixed. The procedure must conventionally be repeated for each color until the multicolor print is completed.

I have now discovered a novel process of multicolor electrostatic printing which does not require repeated charging prior to the application of each color. My process comprises charging the photoconductive surface to a uniform potential, preferably negative, and exposing the charged surface to light through a negative color separation image transparency of a first color to discharge or reduce the potential in areas corresponding to the image of the first color. Then, a developer colored in the first color and carrying a charge of like polarity as that of the surface is applied to the surface. The developer is repelled from the areas of the plate retaining the initial charge to the areas of reduced potential where it adheres forming a visible image of said first color in the areas of reduced potential. Next, without being recharged, the surface is exposed through a negative color separation transparency of a second color to reduce the initial potential or discharge areas corresponding to the second color image and a developer colored in the second color and carrying a charge of like polarity as that of the surface is applied to the surface. The second color developer adheres only to the surface in areas of reduced potential which have been discharged by said second color image transparency.

The novel features of the invention will be more readily understood by referring to the accompanying drawing in which:

FIG. 1 represents a cross-sectional view of an original colored image to be printed,

FIGS. 2 and 3 represent cross-sectional views of yellow and red negative color separation transparencies of the original,

FIG. 4 represents a cross-sectional view of a charged photoconductive plate,

FIG. 5 represents a cross-sectional view of the charged photoconductive plate after it has been exposed to light through the yellow negative color separation transparency of FIG. 2,

FIG. 6 represents a cross-sectional view of the exposed photoconductive plate of FIG. 5 after it has been developed with yellow developer,

FIG. 7 represents the yellow developed photoconductive plate of FIG. 6 after it has exposed, without recharging, to light through the red negative color separation transparency of FIG. 3,

FIG. 8 .represents a cross-sectional view of the exposed photoconductive plate of FIG. 7 after it has been developed with red developer.

In accordance with one preferred embodiment of this invention, a sheet carrying an electrically insulating photoconductive coating of the type described in US. Patent No. 3,052,539 is prepared as follows:

60 grns. of a 10% solution of polyvinyl acetate resin in methanol 40 gms. white zinc oxide, 1 OP. (made by a dry process) 20 ml. of acetone as thinner.

1 (Zinc oxide having a surface pho-toconductivity of at least 10- ohm- /square/watt/cm. when exposed to a wavelength of 3900 A.)

These ingredients were mixed for 1 hour in a porcelain ball mill to obtain a smooth, uniform mix. Ordinary White bond paper is coated with the mix at a coating thickness of 0.5 mils. The coating is air dried.

This paper is charged negatively electrostatically by subjecting it to a corona discharge:

This is effected, for instance, by placing the paper on a flat metal plate electrode and beneath a metal point connected to the negative terminal of a 10 to 20. kilovolt D.C. source, the fiat metal plate being connected to the positive electrode. Typically, the paper is subjected to the corona discharge for a period of 10 seconds when the point is held 2 inches above the paper. For the point there may be substituted a wire or group of wires, or a sharp knife edge of say steel, in which case the distance may be reduced between the wire or knife and the paper and the voltage reduced to give similar electrical field strength in terms of volts per cm., or alternatively a charge may be applied by positioning the paper between condenser plates or the like. The method of charging the paper is fully described in either U.S. Patent No. 2,907,- 674 or Patent No. 3,052,539. The charged paper which may be referred to as the plate has an elevated negative potential. The charged plate is then exposed to the transparencies in a procedure which may be best described in association with the accompanying drawings which represent the procedure diagrammatically. Assuming that the image is to have two colors, yellow and red, negative color separation transparencies are prepared of both the red and yellow images. Using the apparatus described in U.S. 3,052,539, the charged plate is exposed to light through the negative color image transparency for yellow so that the charged plate surface is reduced in negative potential in the yellow image areas. Then using a yellow colored negative developer of the following composition:

Isopar G (An aliphatic hydrocarbon solvent, having a boiling range of 315350 F. and a KB. value of 27.4) 1000.0

(the linseed oil, lead chromate and lead naphthenate are milled for eight hours to form a paste which is then dispersed in the solvent), the exposed paper is developed by submerging the paper in said negative developer in accordance with the procedure set forth in U.S. Patent No. 2,907,604. A yellow image is formed which corresponds to the yellow image which was to be printed. Immediately thereafter, the plate is exposed through the negative color image transparency for red so that the remaining charged plate surface is reduced in negative potential in the red image areas. Then using a red colored negative developer of the following composition:

Paste Gms. Boiled linseed oil 0.75 Cadmium selenide 0.75 Lead naphthenate, dried 0.05

the paste is milled for 5 hours and dispersed in 1000 gms. of Isopar G solvent, the exposed paper is developed by submerging the paper in the red developer in accordance with the previously mentioned procedure. A red image is formed which corresponds to the red image which was to be printed. The resulting plate now has the complete yellow and red image of the original.

It is interesting to note that the red developer does not deposit on the areas already bearing yellow developer. Without commitment on the theory involved, it is believed that this may be in part due to a negative charge on the yellow developer which repels the negatively charged red developer.

The red and yellow image on the plate may then be fixed by conventional means, e.g., heat or the red and yellow developer image may be transferred from the plate to another plain sheet and the transferred developer fixed to the sheet to produce a copy.

The latter transfer method may be used to produce repeated copies from the same original plate. That is the red and yellow developer may be repeatedly applied.

and transferred from the plate. For example, the plate containing the yellow and red image may be used as an image layer in the apparatus described in U.S. Patent No. 2,576,047.

It should be clear that the method of this invention may be used with more than two colors.

While the preferred embodiment of this invention used negatively charged plates, it should be clear that the invention would also be operative with positively charged plates. In the latter case, the developers would have to also be positive in charge so that they would still be repelled from the charged non-image areas.

The developers used in this invention may be any of the conventional developers. Preferably the developers are liquid e.g. in Patent No. 2,907,674 are very desirable. They are pastes or mixtures of either alkyd resins for positively charged developers and linseed oil for negatively charged developers with a pigment dispersed in an insulating liquid of high volume resistivity e.g., toluene, cyclohexane, n-pentane and carbon tetrachloride. Other suitable liquid developers of this type which may be used in the practice of this invention are described in U.S. Patent No. 3,078,231. When using such liquid developers, the simple developing process of Patent No. 2,907,674 may be followed.

Also, the liquid developers of U.S. Patent No. 3,053,- 688, particularly the reversal or negative developers of said patent which are dispersions of polyvinyl chloride and pigment in dimethyl polysiloxane are quite suitable in my invention.

This invention may be practiced using powder developers in place of liquid developers. For example, the procedure for printing the yellow and red image described above may be repeated using apparatus of the U.S. Patent No. 3,052,539 type to develops the image and the following negative or reversal powder developers set forth in U.S. Patent No. 2,986,521:

Gms. 20

In making the developers, the ingredients should be mixed in accordance with the procedure of Patent No. 2,986,521.

While the preferred embodiment of this invention has been described with respect to plates in which the photoconductive layer is zinc oxide dispersed in a film-forming electrically insulating binder, it will be clear that this invention may also be practiced using the widely used selenium plates mentioned previously. It will be obvious to those skilled in the art that other various changes and modifications may be made without departing from the invention, and it is, changes and modifications as fall within and scope of the invention.

What is claimed is:

1. An electrostatic process for printing a multicolor image comprising the steps of charging a photoconductive insulating surface to a uniform potential, exposing the charged surface to light through a negative color separa tion image transparency of a first color to reduce said potential in areas corresponding to the image of the first color, applying to the surface a developer having said first the liquid developers of the type described therefore, aimed to cover all such the true spirit color and carrying a charge of like polarity as that of the surface, whereby said developer adheres to the areas of reduced potential and then without being recharged, exposing said surface to light through a negative color separation image transparency of a second color to reduce said potential in areas corresponding to the second color image and applying to the surface a developer having said second color and carrying a charge of like polarity as that of the surface, whereby said developer of said second color adheres only to the areas of reduced potential corresponding to the image of said second color.

2. The process of claim 1 wherein the surface is charged negatively and both developers are charged negatively.

3. The process of claim 2, wherein the applied developer is subsequently fixed to the surface.

4. The process of claim 2, wherein the applied developer is subsequently transferred from the surface to another surface and fixed to said other surface.

5. The process of claim 2, wherein the developers com-prise dispersions in an electrically insulating liquid of finely-divided negatively charged particles.

6. The process of claim 5 wherein the developer comprises a mixture of pigment and linseed oil dispersed in a liquid to form a dispersion of high volume resistivity, the linseed oil establishing a negative surface charge for the mixture.

References Cited UNITED STATES PATENTS 2,907,674 10/1959 Metcalfe et a1. 11737 2,986,521 5/1961 Wielicki 252-621 3,150,976 9/1964 Johnson 96-12 NORMAN G. TORCHIN, Primary Examiner.

I TRAVIS BROWN, Examiner.

C. E. VAN HORN, Assistant Examiner. 

